This invention pertains to a device for sensing rate of change of current in high amperage highly inductive loads, and more specifically to quench detection devices for superconducting magnet coils.
Large superconducting magnet coils are currently being designed and tested for possible use in fusion reactor devices. These magnets are very expensive. A condition which can damage or destroy such a magnet is the sudden appearance of a zone of normal ohmic resistivity in the otherwise superconducting coil. Such a zone, known as a "quench," quickly heats up due to ohmic heating, causing adjacent zones to revert to normal ohmic resistivity. The overall process is thus self-propagating and can avalanche into a dangerously large, if not complete, loss of superconductivity. This can seriously damage the coil.
Thus, it is necessary that a quench be detected at its onset and that appropriate protective measures be taken. Such protective measures typically include switching energy dump resistors into the magnet circuit, or disconnecting the coil from its power supply.
The impedance of a normal coil has both inductive and resistive (ohmic) components. The impedance of a superconducting coil, on the other hand, is ideally purely inductive. At the onset of a quench, however, the impedance of the superconducting coil acquires a small ohmic component. The ohmic component is much smaller than the inductive component. The challenge in detecting a quench is therefore to detect this small ohmic component in the impedance (and, hence, in the voltage drop across the coil) in the presence of the large inductive component. To oversimplify, this is usually accomplished by developing a signal indicative of the voltage drop attributable to the inductive component of the impedance and subtracting that signal from the total voltage drop or loss. The difference thus calculated corresponds to the ohmic contribution to the voltage loss.
FIG. 1 is a schematic diagram of a Rogowski coil 10, which is a known device for detecting a quench. The Rogowski coil 10 is usually arranged as shown, i.e., enclosing a segment 20 of a primary current carrying element of the magnet system carrying primary current I. A Rogowski coil has the advantage of being able to cancel inductive voltages which originate from ambient magnetic fields other than that originating from the current carrying element. The voltage V.sub.M induced in the Rogowski coil thus indicates the rate of change of current flowing through the primary current carrying element, and is used to develop the signal indicative of the voltage loss attributable to the inductive component of the overall impedance.
While Rogowski coils have served well in measuring currents associated with smaller operating currents, problems have been encountered attempting to scale such coils up to provide a current transformer for operating currents such as are contemplated for fusion applications. First, it is difficult to construct a Rogowski coil of the needed size. Second, even when such a Rogowski coil of the needed size is made, it fails to perform at specified levels.